1. Field of the Invention:
The present invention relates to a drying apparatus whose vane assemblies spiral and push material to be dried against its heat-transmitting surface, and a drying method using the same.
2. Description of Prior Arts:
There are a variety of drying apparatuses for drying different water-containing materials such as fluid material, semi-fluid material or pulverized material.
The applicant of the present application proposed a drying apparatus in Japanese Utility Model Application Laid-Open No. 3-19501. It comprises a cylindrical drying vessel to put material to be dried therein, the inner wall surface of which cylindrical drying vessel makes up a heat-transmitting surface, a heat-generating means encircling the cylindrical drying vessel to transmit heat to the heat-transmitting surface of the cylindrical drying vessel, and a screw-like rotating vane assembly rotatably fixed in the cylindrical drying vessel.
Specifically a rotating axle erects on the bottom of the cylindrical drying vessel, and the screw-like rotating vane assembly is fixed to the rotating axle by a plurality of radial arms, which extend across the material-falling space in the cylindrical drying vessel. After being raised up to the top level of the drying vessel and after being deprived of water content the dried material is allowed to fall in the material-falling space in the cylindrical drying vessel. There is an annular space between the outer vane circumference and the inner wall surface of the cylindrical drying vessel to allow the material to contact the heat-transmitting surface of the cylindrical drying vessel without falling down from the annular space.
In operation the material is spiralled by the rotating vane assembly until it is raised to the top level of the cylindrical drying vessel, then allowing the material to fall on, and again the material is spiralled and raised to the top level. On the way to the top level the material is pushed against the heat-transmitting surface under the influence of centrifugal force, and this up-and-down cyclic motion is repeated until the material has been dried.
This conventional drying apparatus, however, has following defects:
First, in case of drying a material of increased viscosity such sticky material is liable to adhere to the vane sections of the vertical rotary vane assembly and the heat-transmitting surface, thus agglomerating between adjacent upper and lower vane sections to impede the continuous rising of material to be dried. As a result the up-and-down cyclic motion is prevented, and hence unsatisfactory drying results. PA1 Second, the annular space between the outer vane circumference and the inner wall surface of the cylindrical drying vessel extends upward in an elongated spiral form, and foreign substances in the material to be dried are liable to be caught somewhere in the elongated spiral gap, thus preventing rotation of the vertical spiral vane assembly. PA1 Third, the screw-like rotating vane assembly is fixed to the rotating axle by radial arms, which extend across the material-falling space in the cylindrical drying vessel. String- or sheet-like foreign substance such as vinyl sheets when falling down, are likely to be caught by such radial arms, thereby preventing the smooth up-and-down cyclic movement of material in the cylindrical drying vessel. PA1 Fourth, a single spiral vane assembly is used to raise the material to be dried, and therefore, only a limited amount of material can be conveyed for drying, compared with the material remaining on the bottom of the drying vessel, thus causing the lowering of drying efficiency. PA1 Fifth, the areas of the heat-transmitting surface facing the space between adjacent upper and lower vane sections cannot be fully used by pushing material against such areas under the influence of centrifugal force. That is to say, the heat-transmitting surface cannot be fully used. PA1 Sixth, the rotating speed of the spiral vane assembly must be varied with the kinds of material to be dried because otherwise, the cyclic up-and-down movement of material in the drying vessel cannot be caused so as to attain the best drying efficiency. It is, however, difficult to control the rotating speed of the spiral vane to attain the best drying efficiency.